title: Organisation and function of the Phaeospirillum molischianum photosynthetic apparatus.
abstract: We have investigated the organisation of the photosynthetic apparatus in Phaeospirillum molischianum, using biochemical fractionation and functional kinetic measurements. We show that only a fraction of the ATP-synthase is present in the membrane regions which contain most of the photosynthetic apparatus and that, despite its complicated stacked structure, the intracytoplasmic membrane delimits a single connected space. We find that the diffusion time required for a quinol released by the reaction centre to reach a cytochrome bc1 complex is about 260 ms. On the other hand, the reduction of the cytochrome c chain by the cytochrome bc1 complex in the presence of a reduced quinone pool occurs with a time constant of about 5 ms. The overall turnover time of the cyclic electron transfer is about 25 ms in vivo under steady-state illumination. The sluggishness of the quinone shuttle appears to be compensated, at least in part, by the size of the quinone pool. Together, our results show that P. molischianum contains a photosynthetic system, with a very different organisation from that found in Rhodobacter sphaeroides, in which quinone/quinol diffusion between the RC and the cytochrome bc1 is likely to be the rate-limiting factor for cyclic electron transfer.
authors.affiliation: Laboratoire d'Ingénierie des Systèmes Macromoléculaires, UPR 9027, Institut de Biologie Structurale et Microbiologie, CNRS, 31 Chemin Joseph Aiguier, Marseille, 13402, France.
authors.forename: Camille
authors.initials: C
authors.lastname: Mascle-Allemand
authors.forename: Jérôme
authors.initials: J
authors.lastname: Lavergne
authors.forename: Alain
authors.initials: A
authors.lastname: Bernadac
authors.forename: James N
authors.initials: JN
authors.lastname: Sturgis
mesh_terms: D045304:Cytochromes c / Q000378:metabolism; D004058:Diffusion / Q000528:radiation effects; D014450:Electron Transport Complex III / Q000378:metabolism; D004583:Electrons; D006873:Hydroquinones / Q000378:metabolism; D007425:Intracellular Membranes / Q000528:radiation effects / Q000648:ultrastructure; D007700:Kinetics; D008027:Light; D008564:Membrane Potentials / Q000528:radiation effects; D010084:Oxidation-Reduction / Q000528:radiation effects; D019897:Periplasm / Q000528:radiation effects / Q000648:ultrastructure; D010788:Photosynthesis / Q000528:radiation effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D011522:Protons; D011809:Quinones / Q000378:metabolism; D012244:Rhodospirillaceae / Q000378:metabolism* / Q000528:radiation effects / Q000648:ultrastructure; D013347:Subcellular Fractions / Q000378:metabolism / Q000528:radiation effects
chemical_list: D006873:Hydroquinones; D045322:Photosynthetic Reaction Center Complex Proteins; D011522:Protons; D011809:Quinones; D045304:Cytochromes c; D014450:Electron Transport Complex III